Lesson 7: Electron Dot Notation (also see textbook pages 199-200)
In the last two lessons, you learned how to describe the arrangement of electrons using orbital notation (little arrows)
and electronic configuration notation (using superscripts). In this lesson you will learn a third method of describing the
arrangement of electrons which will bring you one step closer to being able to using electron arrangement to help you
predict why some elements are very reactive while others are not. This third method is called electron dot notation.
Electron dot notation utilizes dots to represent those special electrons in an atom that are capable of interacting with
other atoms and possibly forming bonds with those neighboring atoms. The special electrons are known as valence
electrons. The valence electrons are those electrons that are in the outermost orbits or energy levels. Recall from our
discussion of quantum numbers that the principle quantum number tells us the position or energy level on which we can
find traveling electrons. We discussed that the higher the principle quantum number, the farther away from the nucleus
the path or orbit in which the electrons were traveling. Therefore, the electrons that are found in the highest energy
level (greatest principle quantum number) are considered the valence electrons.
Nucleus
Let's look at an example of how to use electron dot notation
to describe the arrangement of electrons about the nucleus
6
1s
of an atom.
Carbon
12.0115
2s
Example 1: Use electron dot notation to describe the arrangement
of electrons in an atom of carbon.
2p
To begin writing electron dot notation, it is essential that you initially
3s
write the electron configuration notation (ECN) for the atom you are
describing. In our example of carbon, the electron configuration
notation for carbon is:
3p
4s
C
x
y
x
y
z
z
C: 1s2 2s2 2p2
Refer to the Board in the diagram above or go back to Lesson 6 if you are uncertain how to write the electron
configuration notations for electrons of an atom.
The next step in writing the electron dot notation for carbon is to analyze the electronic configuration notation to find
the number of valence electrons. Remember, the valence electrons are those electrons in the outermost orbits (energy
levels). In the example of carbon, the outermost orbits filled are 2s and 2p (the highest principle quantum number is 2).
We've drawn a box around those electrons in the notation below.
C: 1s2 2s2 2p2
To determine the number of valence electrons, add up the number of electrons in these outermost orbits. In the
example of carbon, 2+2 = 4. This means that carbon has four valence electrons.
To write the electron dot notation for carbon, first write the symbol for carbon. (Note that no colon is necessary for
electron dot notation!)
Represent each valence electron by making a dot for each valence electron around the clement symbol. It is
convention to place no more than two dots on any one side of the symbol and to evenly distribute the dots around the
symbol as much as possible.
Rule: As you add valence electrons you add a dot to
Nucleus
•
each side (4 sides - include top and bottom of the symbol
• C•
as a "side") of the element symbol. You do not begin to
•
pair dots until all four sides of the elements symbol has a dot.
Go in a clockwise or counterclockwise direction when adding electrons.
12
1s
Mg
Magnesium
2s
Example 2: Write the electron dot notation for magnesium.
2p
Begin by writing the electronic configuration notation for
3s
magnesium. Recall that the atomic number for magnesium is 12
3p
and that the electronic configuration notation for magnesium is:
4s
24.31
x
y
x
y
z
z
Mg: 1s2 2s2 2p6 3s2
Note that the electrons in the outermost orbit would be the 3s electrons.
Mg: 1s2 2s2 2p6 3s2
The number of valence electrons in magnesium would be 2 and we would write the electron dot notation as follows:
•
Mg •
Example 3: Write the electron dot notation for argon.
Begin by writing the electronic configuration notation for argon
here. Refer to your periodic table of elements and your Board
to help you write the electronic configuration, if necessary.
Write the electron configuration notation tor argon here:
__________________________
Nucleus
18
1s
Ar
Argon
2s
2p
39.948
x
y
x
y
z
3s
3p
4s
z
Circle the outermost orbits. How many valence electrons are there?
If you answered eight, you are correct! If not, count again. Note there are 2 electrons in the 3s orbit plus 6 in the filled
3p orbits for a total of eight valence electrons.
Therefore, the dot notation for argon will be: ________
You should have written 8 dots around the symbol Ar with 2 dots on each side of an imaginary square around Ar.)
1. Write the electron dot configuration for the following elements:
Hydrogen, Helium, Oxygen, Boron, Chlorine, Neon, Sodium
2. Write the electron dot configuration for the following elements:
Li, Be, B, C, N, O, f, Ne
3. In problem 2 above, did you notice that each succeeding element added an additional electron? What pattern did
you see develop on the dot diagrams because of this?